Field of The Invention
The present invention relates to a polarizing element for converting a state of polarizing of light, and also to an optical head device incorporating the polarizing element.
Hitherto, a linear retarder such as a 1/4 wavelength plate or a 1/2 wavelength plate has been used as a polarizing element for converting the state of polarization of light in the optical system of a device such as an optical head device. This type of element is adapted to convert the polarization state over its entire area. Display devices also are known such as a liquid crystal display device using twist nematic liquid crystal (referred to as "TN" liquid crystal"). In this type of display device, the state of arrangement of the liquid crystal molecules is varied so as to convert the state of polarization on pixel basis. There is a practical limit in the refining of the pixels. At the present stage, the minimum possible pixel size of 100 to 200 .mu.m square is available. In the field of optical integrated element having light waveguide and grating, there is an increasing demand for a technology which enables, within a small area, conversion from a polarization state having a spatially non-uniform distribution to a uniform polarization state such as linear polarization state, and vice versa.
In particular, in case of an optical integrated element which emits light to the exterior of a wave-guide path by means of a concentric-circle-type grating coupler, it is necessary to conduct a polarization conversion between light having an oscillation plane in a direction tangential to the concentric circle or in a direction of dynamic radius of the circle (that is, light in an inhomogeneous state) and light in a homogeneous state (that is, linear polarized light). Unfortunately, however, there was no element which would conduct such a conversion. The present applicant, therefore, proposed a polarization element having such a conversion function, as well as an optical head device incorporating such a function, in a PCT application No. JP89-00797. These are referred to as "prior art", hereinafter.
The polarizing element and an optical head device of the prior art will be described with reference to FIGS. 1 to 3B.
FIG. 1 is an illustration showing the construction of an optical head device of the prior art. Referring to these figures, a transparent layer 2 of a small refractive index is formed on an Si substrate 1. Transparent layers 3A and 3B of a greater refractive index are formed on the transparent layer 2. The transparent layer 3A has a circular form while the transparent layer 3B has an annular form surrounding the layer 3A. These transparent layers 3A and 3B are insulated from each other. Grating couplers 4A and 4B in the forms of circles concentric with respect to the central axis L are formed on the transparent layers 3A and 3B. A transparent layer 3C of a large refractive index is formed on the surface of the transparent layer 3A with a transparent layer 5A of a small refractive index being sandwiched therebetween. The transparent layer 3C contacts with the transparent layer 3B at a region facing the inner periphery of the grating coupler 4B on the transparent layer 3B. A grating coupler 4C concentric with respect to the central axis L is formed on the surface of the transparent layer 3C. A transparent layer 5B of a small refractive index is formed on the surface of the transparent layer 3B so as to cover the region where the grating coupler 4B exists. The refractice index of the transparent layer 5B is equal to that of the transparent layer 5A. Photo-detectors 6A and 6B are formed on the region which forms the insulation between the transparent layers 3A and 3B on the Si substrate 1. A refractive film 7 is formed in the transparent layer 5A so as to cover the detectors 6A and 6B. The portion of the Si substrate 1 near the central axis L is made hollow by, for example, etching.
The light emitted from a semiconductor laser 8 and linearly polarized is condensed by a condenser lens 9 and is converted by a polarizing element 10A into a light 11 having an electric field vector of a direction tangential or radial to the concentric circles. The light 11 is inputted to the waveguide layer 3C by the concentric grating 4C, so as to become a wave-guided light 12C of a TE (or TM) mode which propagates radially outwardly through the layer 3C. At the outer peripheral region of the waveguide layer 3C, the light 12C is transmitted to the waveguide layer 3B so as to become a wave-guide light 12B. The wave-guided light 12B is then changed into a radiation light 13 through the grating coupler 4B. If the wave-guide light 12B is of the TE mode, the radiation light 13 has such a polarization state that the electric field vector is tangential to the concentric circle (this state will be referred to as "concentric circle polarization"), whereas, if the wave-guided light 12B is of the TM mode, the radiation light is of a polarization state in which the electric field vector is directed in the direction of radius of the concentric circles (this polarization state will be referred to as "radial polarization"). Thus, the light 13 is converged at a point F on the reflective surface 16 of the optical disk 15. The light reflected from the reflective surface 16 is inputted into the waveguide layers 3A, 3B through the grating couplers 4A and 4B so as to become, respectively, wave-guided lights 18A and 18B of TE (or TM) mode propagating radially outwardly and inwardly through the respective layers 3A and 3B. The wave-guided lights 18A and 18B are radiated at the radially outer and inner ends of the waveguide layers 3A and 3B, and are received by the photo-detectors 6A and 6B.
In this prior art arrangement, in order to attain a large convergence of light, a polarizing element 10B, which is capable of performing a reversible conversion between linear and concentric circle (or radial) polarization states, is disposed between the point of convergence F and the grating coupler 4B. A polarizing element 10A, capable of performing a reversible conversion between the linear and concentric circle polarization states is disposed between the semiconductor laser 8 and the grating coupler 4C, in order to attain a higher degree of input coupling efficiency through the grating coupler 4C.
A description will now be made of the constructions of the polarizing elements 10A, 10B in the prior art, as well as the principle of conversion between the concentric circle polarization state and the linear polarization state, with specific reference to FIGS. 2a and 2b. FIG. 2a shows a first embodiment described in the specification of the aforementioned PCT application disclosing the prior art. A polarizing element 210 is composed of a 1/4 wavelength plate 221 and a homogeneous liquid crystal layer 224. The surfaces 225, 226 of transparent substrates 222, 223 have been subjected to a liquid crystal orientation treatment in the directions 214A, 214B, 214C and 214D which are inclined at 45.degree. to the directions tangential to the concentric circles. A tangential light 213 (in polarization directions 213A, 213B, 213C, 213D) is a light having polarization directions which form 45.degree. to the directions 214 (214A, 214B, 214C, 214D) of the optical axes (directions of liquid crystal orientation) of a homogeneous liquid crystal layer 224. This light 213, as it passes through the homogeneous liquid crystal layer 224 having an optimum thickness, is changed into light 212 of a circular polarization having polarization directions 212A, 212B, 212C, 212D). The light is then changed to a linearly polarized light 211 as it passes through the 1/4 wavelength plate 221. FIG. 2b shows another embodiment of the prior art shown in the above-mentioned PCT application. Referring to this Figure, the polarizing element 21 is composed of a 1/4 wavelength plate and two liquid crystal layers: namely, a TN type liquid crystal layer and a homogeneous liquid crystal layer. The homogeneous liquid crystal layer 22 is interposed between transparent substrates 21A and 21B, while the TN liquid crystal layer 23 is interposed between the transparent substrate 21B and a transparent substrate 21C. The homogeneous liquid crystal layer 22 is oriented concentrically or radially on the surfaces of the transparent substrates 21A, 21B, so that its optical axis is tangential or radial as denoted by 22A. On the other hand, the TN liquid crystal layer 23 has been orientation-treated such that the orientation is tangential at one of the orientation surfaces and 45.degree. to the tangent at the other orientation surface. Therefore, the polarization direction turns 45.degree. in the clockwise direction for example, as the light passes through this liquid crystal layer 23. Consequently, a light 20 of concentrical circle polarization (polarizing directions 20A, 20B, 20C, 20D) is changed into a light 24 of polarization (polarizing direction 24A, 24B, 24C, 24D). In addition, the homogeneous liquid crystal layer 22 adds a phase difference of 1/4 wavelength, whereby a circularly polarized light 25 is obtained. This circularly polarized light 25 is then changed to a linearly polarized light 26B as it passes through a 1/4 wavelength plate 26 the optical axis of which extends in the direction 26A. Conversely, a linearly polarized light, when passed through a 1/4 wavelength plate 26 and this polarizing element 21, is converted to a concentric circle polarizing light or radially polarized light due to reversability of the light. This polarizing element can produce a phase delay, i.e., aberration, in the light passing through the liquid crystal layer. In the prior art, thickness of the transparent substrates 222, 223 or 21A, 21B, 21C are suitably modulated depending upon positions thereon to cancel this aberration, thereby attaining a complete polarization without aberration.
A description will now be given of effects produced by the polarization element 10B which is interposed between the grating 4B and the point of convergence F as shown in FIG. 1.
FIGS. 3A and 3B show examples of light-intensity distribution cross-sections at the point of convergence on which a concentric circle-polarized (or radially polarized) light and a linearly polarized light, respectively, are converged through a concentric circular grating coupler. As will be understood from a comparison between FIGS. 3a and 3b, the concentric-circle polarized light (or radially polarized light) exhibits an inferior converging characteristic as compared with the linearly polarized light. This is because, in the former case, the electric field components at diagonally opposing portions of the concentric circle for vectors are reverse to each other so as to cancel each other at the point of convergence F. The polarization element 10b disposed between the grating coupler 4B and the point of convergence F offers a good converging characteristic as it converts the concentric-circle polarized light (or radially polarized light) into a linearly polarized light.
On the other hand, the input coupling efficiency of the grating couplers 4A, 4B for introducing the reflected light 17 into the wavegudie paths 3A, 3B is related to the state of polarization of the reflected light. In order to attain a high coupling efficiency, it is desirable that the reflected light has the same state of polarization as the radiation light. Thus, the polarization element 10B, which conducts a reversible polarization between linearly polarized light and concentric-circle (or radial) polarized light can also produce an effect to improve the efficiency of coupling of the reflected light to the wave-guided light.
The polarization element 210 or 21 shown in FIGS. 2a and 2b of the prior art, however, suffers from the following disadvantages. When a circularly polarized light passes through the homogeneous liquid crystal layer 224 or 22, the phase of the light is offset by an amount .PHI. at the position of the deflection angle .PHI.. It is therefore necessary to cancel this phase delay (aberration) by, for example, modulating the thickness of the transparent plate, which require greater number of parts and process steps. In addition, the construction of this polarization element is complicated as it is composed of a 1/4 wavelength plate and one or two liquid crystal layers. This makes it difficult to design and construct a compact element. Furthermore, liquid crystal orientation in directions 45.degree. to the lines tangential to the concentric circles cannot be attained by a simple orientation treatment such as rubbing.